The present invention relates to an etchant for use in patterning thin metal films by wet etching and to a method of etching with the same. More particularly, the invention relates to an etchant and an etching method for use in the production of semiconductor devices such as semiconductor elements and liquid-crystal display elements.
Recently, electrode and gate wiring materials for use in semiconductor devices such as semiconductor elements and liquid-crystal display elements are increasingly required to have a higher degree of precision in microfabrication. It has been proposed to use metallic materials having a lower resistance. Examples of metallic materials having a low resistance include aluminum and aluminum alloys, and these materials are coming to be used increasingly.
Examples of techniques for processing a thin film of such a metal to form a pattern of a microstructure such as a wiring include wet etching techniques in which a photoresist pattern formed on the surface of a thin metal film by photolithography is used as a mask to conduct etching with a chemical to thereby pattern the metal film, and further include dry etching techniques such as ion etching and plasma etching.
Compared to dry etching techniques, the wet etching techniques are economically advantageous in that they do not necessitate expensive apparatus and relatively inexpensive chemicals are used. In addition, by the wet etching techniques, substrates having a large area can be evenly etched while attaining high productivity per unit time. Because of these, the wet etching techniques are frequently used as a process for producing a thin-film pattern.
During such processing for wiring formation, there are cases where aluminum and aluminum alloys develop hillocks (blisterlike projections generating on aluminum surfaces upon heat treatment) in a heat treatment step, e.g., substrate heating in film deposition in a process for semiconductor device production. The generation of hillocks makes it difficult to superpose an insulating layer on the aluminum wiring. Namely, it has been known that even when an insulating layer is formed on the aluminum wiring having hillocks on its surface, the hillocks remain penetrating through the insulating layer, resulting in insulation failures. The protruding parts of the hillocks cause short-circuiting when they come into contact with another conductive thin film.
It is also known that when aluminum or an aluminum alloy is used as a wiring material and this wiring is directly contacted with ITO (indium oxide-tin oxide alloy) as a transparent electrode, then an altered layer is formed in that surface of the aluminum or aluminum alloy which is in contact with the ITO and, as a result, the contact part has increased contact resistance.
For preventing the hillock generation and altered-layer formation described above, various multilayer wirings have been proposed which comprise a layer of aluminum or an aluminum alloy and, superposed thereon, a layer of a different metal, e.g., a layer of a high-melting metal such as molybdenum, a molybdenum alloy, or chromium (see, for example, JP-A-9-127555, JP-A-10-256561, JP-A-2000-133635, JP-A-2001-77098, and JP-A-2001-311954). (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d.)
However, in the wet etching of multilayer films, some combinations of metals have resulted in exceedingly low production efficiency because of the necessity of successively etching the individual layers constituting the multilayer film with two different etchants. It is known that even when an etchant with which all layers constituting a multilayer film can be simultaneously etched is used, cell reactions occur due to contact with each of the layers of different metals, resulting in a different etching behavior, such as a higher etching rate than in the case of single-layer etching. (See, for example, SID CONFERENCE RECORD OF THE 1994 INTERNATIONAL DISPLAY RESEARCH CONFERENCE, p.424.)
Consequently, a difference in etching rate arises between the metal layers, and this may result in under cutting in the lower metal layer (the phenomenon in which the lower metal layer is etched more quickly than the upper metal layer to leave overhangs of the upper metal layer) or side etching in the upper metal layer (the phenomenon in which the upper metal layer is etched more quickly than the lower metal layer). In particular, when under cutting has occurred, covering with a gate insulating film (e.g., SiNx) in the overhang parts may be insufficient because the multilayer film after the etching has a profile which is not tapered, resulting in insulation resistance failures, etc.
For eliminating the problem described above, a multilayer wiring has been proposed which is formed from a multilayer film comprising a first layer made of aluminum or an aluminum alloy disposed on a surface of an insulating substrate and a second layer formed over the first layer and containing at least one impurity selected from nitrogen, oxygen, silicon, and carbon. (See, for example, JP-A-11-284195.)
Such multilayer films attain excellent productivity because only one kind of metal target is necessary for film deposition. This is because the second layer in this kind of multilayer film can be formed by: reactive sputtering in which a gas such as, e.g., N2, O2, or CO2 is incorporated into a thin metal film deposited by sputtering or the like; plasma treatment in which the same gas as any of these is used; or a method in which a silicon-containing film such as SiNx or SiOx is formed on the surface and the resultant coating is annealed. Since this second layer can prevent the generation of hillocks and enables the formation of a multilayer film having excellent corrosion resistance, this technique is economically superior to the case in which films are separately formed with different metal targets.
Examples of general etchants for the second layer in such multilayer films, e.g., aluminum nitride or an aluminum nitride alloy, include aqueous sodium hydroxide solutions and hot phosphoric acid. Known as general etchants for aluminum or aluminum alloys are aqueous solutions having a phosphoric acid content of 70% by weight or higher and containing nitric acid and acetic acid.
However, use of alkaline etchants such as aqueous sodium hydroxide solutions has had an intrinsic problem that the photoresist resin layer patterned by photolithography dissolves away.
Furthermore, in the case where a multilayer film comprising a first layer made of aluminum or an aluminum alloy and a second layer formed thereon which is made of aluminum or an aluminum alloy each containing at least one of nitrogen, oxygen, silicon, and carbon is etched with any of the general etchants for aluminum (or alloys thereof), there has been a problem that the etching rate considerably differs between the constituent layers.
Specifically, not only the second layer (upper layer) has a lower etching rate even in single-layer etching, but also use of this etchant causes cell reactions with the superposed layers to further heighten the rate of etching of the first layer (lower layer). Because of this, under cutting is unavoidable in this etching and it has hence been extremely difficult to etch the superposed upper and lower layers so as to form a fine wiring line profile with high precision.
An object of the invention, which has been achieved under those circumstances, is to provide an etchant and etching method with which, when a multilayer film comprising a first layer made of aluminum or an aluminum alloy having a low resistance and a second layer formed thereon containing at least one of nitrogen, oxygen, silicon, and carbon is used as a wiring material, a fine wiring line profile with satisfactory precision can be obtained through one operation of etching the multilayer film while inhibiting under cutting (formation of overhangs).
The present inventors made intensive investigations in order to overcome the problems described above.
As a result, it has unexpected been found that such a multilayer film can be etched through one etching operation so as to have a desired wiring line profile when an etchant is used which contains phosphoric acid and nitric acid and has a composition in a specific range wherein the phosphoric acid content is lower than in existing etchants of this kind.
Specifically, the inventors have found that when an etchant which has a phosphoric acid content of from 35 to 65% by weight and a nitric acid content of from 0.5 to 15% by weight and which preferably further contains from 1 to 15% by weight acetic acid or contains from 0.1 to 15% by weight aliphatic or aromatic sulfonic acid and/or salt thereof is used for etching such as multilayer film, then this multilayer film can be etched, through one etching operation, to form a desired wiring line profile, e.g., a tapered shape, while inhibiting under cutting. The invention has been thus completed.
The invention provides an etchant for a multilayer film comprising a first layer made of aluminum or an aluminum alloy having formed thereon a second layer made of aluminum or an aluminum alloy each containing at least one element selected from nitrogen, oxygen, silicon, and carbon, the etchant having a phosphoric acid content of from 35 to 65% by weight and a nitric acid content of from 0.5 to 15% by weight. The invention further provides a method of etching with this etchant.